1. Field of the Invention
The present invention relates to an improved, reliable photoelectric conversion device and an improved, reliable photoelectric conversion module which excel in weatherability, heat resistance and abrasion resistance and continuously exhibit a desirable photoelectric conversion efficiency without being deteriorated even upon repeated use over a long period of time under severe environmental conditions of high temperature and high humidity. More particularly, the present invention relates to an improved photoelectric conversion device and an improved photoelectric conversion module each having a photovoltaic element comprising a semiconductor active layer and a transparent and conductive layer disposed in this order on an electroconductive substrate, and a surface protective member disposed on said photovoltaic element, said surface protective member including a transparent material member composed of a specific fluorine-containing polymer resin, wherein the photovoltaic element of the photoelectric conversion device or photoelectric conversion module is tightly sealed by means of the aforesaid specific surface protective member with an improved adhesion between the photovoltaic element and the surface protective member. Hence, the photoelectric conversion device or photoelectric conversion module excels in weatherability, moisture resistance, heat resistance and abrasion resistance and continuously exhibits a desirable photoelectric conversion efficiency without being deteriorated even upon repeated use over a long period of time under severe environmental conditions of high temperature and high humidity.
2. Related Background Art
In recent years, heating of the earth because of the so-called greenhouse effect due to an increase of atmospheric CO.sub.2 has been predicted. In view of this, there is an increased demand for a means of power generation capable of providing clean energy without causing CO.sub.2 buildup. In this regard, nuclear power generation has been considered to be advantageous in view of not causing CO.sub.2 buildup. However, there are problems for nuclear power generation in that it unavoidably produces radioactive wastes which are harmful for living things and there is a probability that leakage of injurious radioactive materials from the nuclear power generation system will happen when the system is damaged. Therefore, there is an increased societal demand for early realization of a power generation system capable of providing clean energy without causing CO.sub.2 buildup as in the case of thermal power generation and without causing radioactive wastes and radioactive materials as in the case of nuclear power generation.
There have been various proposals which are expected to meet such societal demand. Among those proposals, solar cells (photoelectric conversion elements in other words) are expected to be a future power generation source since they supply electric power without causing those problems as above mentioned.
There have been proposed a variety of solar cells for commercial and home appliances. These solar cells include single crystal silicon solar cells, polycrystal silicon solar cells, amorphous silicon solar cells, copper indium selenide solar cells, and compound semiconductor solar cells. Of these solar cells, various studies have been made on so-called thin film crystal silicon solar cells, compound semiconductor solar cells and amorphous silicon solar cells since their semiconductor active layer can be relatively easily formed in a large area and in a desired form and they therefore can be easily produced at a relatively low production cost.
Particularly, thin film amorphous solar cells, specifically, amorphous silicon solar cells, comprising a metal substrate, an amorphous silicon semiconductor active layer disposed on said metal substrate, and a transparent and conductive layer disposed on said semiconductor active layer have been evaluated as being the most advantageous among the conventional solar cells because their semiconductor active layer comprised of amorphous silicon (hereinafter referred to as a--Si) can be easily formed in a large area and in a desired form on a relatively inexpensive substrate with a low production cost and they are light and excel in shock resistance and flexibility, and in addition, they can be designed into a solar cell module in a desired configuration which can be used as a power generation source.
Now, in the case of an amorphous silicon solar cell having a semiconductor active layer comprising, for example, an a--Si thin film disposed on a glass plate as a substrate, light is impinged through the substrate side, and because of this, the glass plate can be designed to serve as a protective member. However, in the case of the aforementioned solar cell having the a--Si semiconductor active layer disposed on the metal substrate, because the metal substrate does not permit incident light to transmit therethrough, light is impinged through the side opposite the metal substrate, and therefore, it is necessary to dispose an appropriate transparent protective member on the side through which light is impinged such that it protects the solar cell element. In the conventional manner to do this, a transparent fluorine-containing polymer film comprised of fluororesin or a fluororesin-containing composition is used as the surface protective member and a transparent thermoplastic resin is used as a filler under the transparent fluorine-containing polymer film.
In fact, the fluorine-containing polymer film is often used in the above described manner, since it is advantageous in that it is satisfactory in terms of weatherability and water-repellency and serves to diminish a deterioration in the photoelectric conversion efficiency of the solar cell element caused due to a reduction in the transmittance of the surface protective member which occurs when the protective member is yellowed or clouded as a result of the protective member having been deteriorated. As for the thermoplastic resin used as the filler in combination with the fluorine-containing polymer film, it is also often used since it is relatively inexpensive and suitable for protecting the solar cell element.
Now, description will be made of the conventional solar cell module with reference to FIG. 1.
FIG. 1 is a schematic cross-sectional view of an example of the conventional solar cell module. In FIG. 1, reference numeral 701 indicates a transparent surface protective film comprising a fluorine-containing polymer thin film, reference numeral 702 indicates a transparent thermoplastic resin as a filler, reference numeral 703 indicates a photovoltaic element (or a solar cell), and reference numeral 704 indicates an insulating member.
In this solar cell module, the filler disposed on the rear side of the photovoltaic element 703 is comprised of the same thermoplastic resin disposed on the side through which light is impinged.
Specific examples of the fluorine-containing polymer thin film as the transparent surface protective film 701 are fluororesin films such as ETFE (ethylene-tetrafluoroethylene copolymer) film, PVF (polyvinyl fluoride) film, and the like. Specific examples of the transparent thermoplastic resin as the filler 702 are EVA (ethylene-vinyl acetate copolymer), butyral resin, and the like.
The insulating member 704 is disposed in order to reinforce the solar cell module while adding an appropriate rigidity thereto. The insulating member 704 is usually comprised of an organic resin film such as nylon film, TEDLAR (trademark name, laminated aluminum foil), or the like.
The thermoplastic resin 702 serves as an adhesive between the photovoltaic element 703 and the surface protective film 701 (that is, the fluororesin film) and also as an adhesive between the photovoltaic element and the insulating member 704. In addition to this, the thermoplastic resin 702 also serves as a filler for preventing the photovoltaic element from being externally damaged and from being damaged from external shock.
Incidentally, the present inventors made experimental studies of the thermoplastic resin as the filler in the conventional solar cell module thus constituted by way of a so-called acceleration test corresponding to exposure in outdoors over a long period of time (more than 20 years). As a result, there were obtained the following findings. That is, when the solar cell module is continuously exposed to sunlight in outdoors over a long period of time (for example, over 20 years), the thermoplastic resin as the filler is liable to suffer from gellation whereby it becomes clouded or it is liable to be yellowed due to an increase in conjugated double bonds in the chemical structure of the resin. The occurrence of such cloudiness or yellowing makes the thermoplastic resin as the filler to become poor in light transmittance, resulting in reducing the photoelectric conversion efficiency of the solar cell module. Hence, the thermoplastic resin as the filler is not sufficient enough not only in terms of weatherability but also terms of durability upon repeated use in outdoors over a long period of time (for example, over 20 years).
The present inventors also found that the above problems become significant when the solar cell module is continuously used in a severe outdoor atmosphere of high temperature and high humidity, wherein it is placed at a roof of a building or it is integrated with the roof.
Further, the present inventors noted a problem for the solar cell module in which the filler is comprised of EVA. That is, when this solar cell module is continuously used at a temperature of 80.degree. C. or above, the filler is liable to be remarkably yellowed.
In addition, the present inventors obtained a solar cell module in which the filler is comprised of butyral resin. When this solar cell module is continuously used outdoors, moisture is liable to invade through end portions of the filler into a defective portion of the photovoltaic element since the butyral resin is relatively high in hygroscopicity. In this case, the invaded moisture and the electric field of the photovoltaic element cause the constituent metal of the collecting electrode of the photovoltaic element to become ionized to cause a precipitate, resulting in growing an independent metal layer. When this phenomenon proceeds, a short circuit occurs in the photovoltaic element wherein an electric charge produced cannot be effectively outputted, resulting in reducing the photoelectric conversion efficiency of the photovoltaic element. In addition to this, the butyral resin has an inherent problem of causing a so-called devitrification phenomenon in that its transparency is markedly reduced when it is exposed to an atmosphere of high temperature and high humidity.
Now, Japanese Laid-open patent application No. 76229/1992 (hereinafter referred to as Japanese patent document) discloses a solar cell module which is free of coloring (for example, yellowing) in the constituent resin and which is slightly deteriorated in the severe high temperature-high humidity test. The Japanese patent document describes that a protective film composed of a resin containing perfluoroalkylene group and active hydrogen (i.e., LUMIFLON (trademark name, produced by Asahi Glass Co., Ltd.)) is disposed in a CdS/CdTe type solar cell comprising a CdS/CdTe semiconductor layer disposed on a substrate. As for the LUMIFLON used, the Japanese patent document describes that it is a fluorine polymer having a number average molecular weight of 20,000 to 80,000 and containing perfluoroalkylene group and pendant active hydrogen and which can produce a crosslinked polymer when reacted with melamine or a compound having an isocyanate group (that is, a crosslinking agent). Further, the Japanese patent document describes that a protective film excelling in moisture resistance is obtained when the LUMIFLON is crosslinked with isocyanate or resol type phenol resin.
It is understood that the Japanese patent document is directed to a technique of merely disposing the aforesaid protective film on a thin film type solar cell element.
In the solar cell module described in the Japanese patent document, there still remain some problems to be resolved particularly in terms of resistance to change in environments and durability. Specifically, the formation of the foregoing protective film in the Japanese patent document is conducted by applying a coating composition containing the foregoing resin by means of a screen printing technique and hardening the coating composition applied at an elevated temperature. Because of this, it is difficult to attain a sufficient thickness for the protective film to be formed, wherein when the thin film solar cell element has an uneven surface, although the protective film can be formed along such uneven surface, the resultant is such that is accompanied by irregularities at the surface. Thus, the product is problematic in that it is not satisfactory particularly in terms of mechanical resistance since the surface thereof is liable to be externally damaged because of the irregularities present at the surface.
In view of avoiding the occurrence of such problems the use of a glass member as a protective member for a thin film solar cell element is the most appropriate. In fact, there are a number of proposals for sealing a thin film solar cell element by using a glass member. However, the glass sealing is difficult to attain desirable flexibility, shock resistance, lightweight, and production cost reductin for a solar cell module obtained.